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| Titel |
Air-sea interface in hurricane conditions |
| VerfasserIn |
Alexander Soloviev, Silvia Matt, Atsushi Fujimura |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250053968
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| Zusammenfassung |
| Improving hurricane prediction models requires better understanding of complex processes
taking place in the oceanic and atmospheric boundary layers at high wind speeds. The role of
sea spray in this process is not yet completely understood. Estimates provided in Soloviev
and Lukas (2006) suggest that, in the framework of existing sea spray generation functions
(see e.g., Andreas 2004), the reduction of turbulent friction due to buoyancy effects
associated with the entrainment of spray droplets in the airflow has a relatively small effect
on the drag coefficient. Furthermore, the wind loses a part of its momentum to
accelerate the spray, which should result in a slow increase of the drag coefficient with
wind speed rather than its decrease. In this work, we further develop the hypothesis
formulated in Soloviev and Lukas (2006) that the change of the air-sea interaction
regime in hurricane conditions is associated with the mechanism of direct disruption
of the air-sea interface by pressure fluctuations working against surface tension.
This is achieved through the Kelvin-Helmholtz (KH) type instability of the air-sea
interface. This instability initiates the tearing of short wavelet crests, subsequent
smoothing of the sea surface, and detachment of the airflow from waves, which
reduces the drag coefficient at the air-sea interface (Powell et al. 2003; Donelan et al.
2004; Black et al. 2007). A nondimensional number K = u*a-(gÏăsÏw-Ï2a)1-4 is
the criteria for the KH instability (Soloviev and Lukas 2010). (In this formula u*a
is the friction velocity from the air side, g the acceleration due to gravity, Ïătthe
surface tension, Ïw and Ïa are the water and air density, respectively.) In order to
investigate the mechanism of the disruption of the air-sea interface, we have conducted a
series of numerical experiments using the computational fluid dynamics software
ANSYS Fluent. The 3D experiments were initialized with either a flat interface or
short wavelets and wind stress was applied, ranging from zero stress to hurricane
force stress. The wind stress was applied either at the upper boundary of the air
layer or through the horizontal pressure gradient. The disruption of the air-water
interface resembling the KH instability is observed when wind reaches hurricane
force. The numerical experiments with imposed short wavelets demonstrate the
tearing of wave crests, formation of water sheets and spume ejected into the air, and
smoothening of the water surface. In conclusion, a conceptual framework for merging the
effects of the two-phase environment with the contribution to the drag from waves is
discussed.
References:
Andreas, E.L., 1998: A new sea spray generation function for wind speeds up to 32 m
s-1. J. Phys. Oceanogr. 28, 2175-2184.
Black, P.G., E. A. D’Asaro, W.M. Drennan, J. R. French, P. P. Niiler, T. B. Sanford, E.J.,
Terrill, E. J. Walsh, and J. A. Zhang , 2007: Air-Sea Exchange in Hurricanes: Synthesis of
Observations from the Coupled Boundary Layer Air-Sea Transfer Experiment. Bulletin of the
American Meteorological Society 88(3): 357-374.
Donelan, M. A., B. K. Haus, N. Reul, W. Plant, M. Stiassnie, H. Graber, O. Brown, and E.
Saltzman, 2004. On the limiting aerodynamic roughness of the ocean in very strong winds.
Geophysical Research Letters 31, L18306.
Powell, M.D., P.J. Vickery, and T.A. Reinhold, 2003: Reduced drag coefficient for high
wind speeds in tropical cyclones. Nature 422: 279-283.
Soloviev, A., and R. Lukas, 2006: The Near-Surface Layer of the Ocean: Structure,
Dynamics, and Applications. Springer, 572 pp.
Soloviev, A., and R. Lukas, 2010: Effects of bubbles and spray on air-sea exchange in
hurricane conditions. Boundary-Layer Meteorology 136, 365-376. |
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